This invention relates to a mobile radio communication system in which base stations are centrally installed in a plurality of geographically adjacent service areas to establish communication via a closest base station between a mobile unit and a called subscriber, and particularly to such a system in which the transmitter output capacity of each mobile unit may be greatly reduced.
Generally, in a "cellular" or mobile phone system of this type a base station is installed in the center of each service area, and is equipped with transmitters having sufficient output capacity or range to reach the entire service area, and high sensitivity receivers. Each mobile unit is also provided with a transmitter of relatively high output and a high sensitivity receiver. In this manner, a communication link may be established between the mobile unit and the base station in the service area in which the mobile unit is traveling.
FIG. 1 shows a prior art arrangement in which base stations 1a, 1b and 1c are centrally installed in geographically adjacent, hexagonal service areas A, B and C. Although hexagonal areas are shown for simplicity, they would actually be more in the nature of overlapping circles as illustrated by the broken lines in the figure. The base stations have omnidirectional antennas. The dimensions of the service areas are defined or determined by the gain of the base station antennas, the output of the transmitters, the sensitivity of the receivers, etc. A mobile unit 2 traveling in service area A, for example, communicates with a called party or subscriber via base station 1a.
More specifically, a signal transmitted from the antenna of the mobile unit 2 is received by the antenna at the base station 1a for service area A, and a called subscriber is accessed by a switchboard (not illustrated) or the like. Once the link is established, return signals from the subscriber are transmitted to the mobile unit 2 via the base station 1a. In this manner telephone service is established between the mobile unit 2 and the subscriber.
In such a direct link prior art system, the transmission output of the mobile unit is smaller than that of the base station. To supplement this difference the base station embodies a highly sensitive receiving system such as a space diversity receiver, a sectarized receiver, etc. The transmission output of the mobile unit is generally only 1/25-1/160 of the transmission output of the base station; the mobile unit output is typically 0.6-4 W when the base station output is 100 W.
With such a mobile radio communication system a relatively large output is thus required for the mobile unit transmitter, and as a result power consumption is inevitably increased. In particular, the battery capacity must be comparatively large, and a small-sized and lightweight portable mobile phone is difficult to manufacture.